1. Field of the Invention
Fresh market availability of small fruits such as raspberries (Rubus idaeus L.) and strawberries (Fragaria ananassa, Duchesne) is restricted due to rapid deterioration, primarily the result of fruit senescence and diseases after harvest. Several methods for prolonging shelf-life have been used, including harvesting at earlier stages of maturity than are ideal for consumption (T. M. Sjulin et al., 1987, J. Amer. Soc. Hort. Sci. 112(3: 481-487); gamma irradiation (P. Thomas, 1986, CRC Crit. Rev. Food Sci. Nutrit. 24: 357-400); controlled atmospheres and temperatures (B. L. Goulart et al., 1992, J. Amer. Hort. Sci. 117(2): 265-270; N. F. Sommer, 1985, Can. J. Plant Pathol. 7(4): 331-339); and biological control (W. Janisiewicz, 1988,"Biological Control of Diseases of Fruit," In Biocontrol of Plant Diseases, Vol. 2, K. G. Mukerji and K. L. Garg (ed.), pages 153-165, CRC Press, Boca Raton, Fla.).
This invention relates to a method of using a natural compound, released during ripening by certain fruits, for the postharvest control of fungal decay in small fruit and berries.
2. Description of the Prior Art
Gamma radiation has been examined for use in the extension of the shelf life of perishable fruits (Thomas, supra, 1986). Two major obstacles in the widespread commercial use of gamma radiation to treat perishable fruit have been lack of consumer acceptance and damage to fruit at dosage levels sufficient to effectively control pathogens. Additionally, irradiated foods have been shown to contain and/or release a myriad of radiolytic products, many of which have high mammalian toxicities or contribute to off-flavors (W. W. Nawar, 1986, Food Rev. Int. 2(1): 45-78). Modified atmosphere packaging (MAP) using plastic films decreased strawberry fruit decay but contribute to off-odors and flavors (M. Shamaila et al., 1992, J. Food Sci. 57(5): 1168-1172, 1184). Shamaila et al. found that unpackaged strawberries had the highest overall levels of desirable sensory attributes at all storage times tested, but the berries were infected with fungi (species unreported) after 6 days. Therefore, it would seem that if normal O.sub.2 and CO.sub.2 levels could be maintained while fungal growth was being prevented, optimum sensory qualities would be preserved.
The principal method of controlling postharvest diseases of these fruit is via the suppression of inoculum production and subsequent infection of the flowers and developing fruit (J. W. Eckert et al., 1988, Ann. Rev. Phytopathol. 26: 433-469). Several fungal species (including Alternaria alternata, Botrytis cinerea and Colletotrichum spp.) constitute the majority of postharvest pathogens on small fruit and berries (Eckert et al., supra).
Preharvest applications of fungicides have been used to control postharvest fungal decay (J. A. Freeman et al., 1977, Can. J. Plant. Sci. 57(1): 75-80); however, the use of certain fungicides to control grey mold caused by B. cinerea has increased the frequency of diseases caused by Mucor spp. and Rhizopus stolonifer (Eckert et al., supra). Additionally, strains of B. cinerea have developed resistance to several different classes of fungicides (Eckert et al., supra; R. J. Vali et al., 1992, Plant Dis. 76(9): 919-924). The potential for postharvest application of fungicides is limited by both adverse effects due to wetting of the fruit and by stringent federal and state regulations concerning the use of currently-available fungicides (Eckert et al., supra, 1988).
Red raspberries and strawberries release a myriad of volatile compounds during ripening (T. Hirvi, 1983, Lebensm.-Wiss. u.-Technol 16(3): 157-161; T. Hirvi et al., 1982, Z. Lebensm. Unters. Forsch. 175(1): 113-116; E. Honkanen et al., 1980, Z. Lebensm. Unters. Forsch. 171(2): 180-182; M. Larsen et al., 1990, Z. Lebensm. Unters. Forsch. 191(21): 129-131; T. Pyysalo, 1976, Z. Lebensm. Unters. Forsch. 162(3): 263-272; T. Pyysalo et al., 1979, J. Agr. Food Chem 27(1): 19-22). Many of these compounds have been shown to have antifungal activities (N. Fries, 1973, Trans. Brit. Mycol. Soc. 60(1): 1-21; A. Paul1 et al., 1987, Z. Lebensm. Unters. Forsch. 185(1): 10-13; R. S. Farag et al., 1989, J. Food Sci. 54(11): 74-76; H. Hitokoto et al., 1980, Appl. Environ. Microbiol. 39(4): 818-822; C. L. Wilson et al., 1987, Plant Dis. 71(4): 316-319). Volatile C.sub.5 -C.sub.9 aldehydes occurring in mature citrus fruit were found to inhibit Penicillium digitatum (P. L. Davis et al., 1972, Phytopathology 62: 488-489). The natural volatile benzaldehyde has been reported to protect peaches from Rhizopus rot (C. L. Wilson et al, 1989, Ann. Rev. Phytopathol. 27: 425-441). Acetaldehyde vapor has been shown to decrease decay in raspberries and strawberries (E. Pesis et al., 1990, J. Sci. Food Agric. 52: 377-385; K. Prasad et al., 1973, Plant Disease Reporter 57: 795-797; K. Prasad et al., 1974, Phytopathology 64: 948-951). Although many of these compounds are effective fungal inhibitors at relatively low concentrations, at present none are used commercially to prevent or delay fruit decay. These natural volatile compounds may function as effective antifungal agents if sufficient concentrations can be maintained in the gas headspace surrounding the fruit.